Recently, in semiconductor fabrication technology, growth preventing masks are arranged on both sides of a stripe-shaped aperture, and a semiconductor layer is selectively epitaxial-grown on the aperture by MOVPE (Metalorganic Vapor Phase Epitaxy), which is called selective MOVPE. By this technology, a light waveguide device can be formed without an etching technique, therefore the fabrication can be simplified and the yield gets better. When the selective MOVPE technology is applied to epitaxial growth of a quantum well structure of III-V family of compound semiconductor, diffusion of growth species (mainly III family of organic metal material) in vapor phase depends on the width of masks by which the solid phase composition of the growth species varies, and the growth velocity depends on the width of the mask by which the width of the quantum well structure layer varies. Based on the synergistic effect of the above mentioned theory, bandgap energy (transmission energy in the first energy level between valence electron band and conduction band) on the same substrate can be changed by a single MOVPE growth process using a mask with different width partially. This kind of technology is good for fabrication of a semiconductor photonic integrated circuit which is required monolithic integration of optical function devices having different bandgap energy. The inventor has proposed an integrated light source and a tunable DBR (Distributed Bragg Reflection) laser each of which is fabricated by monolithic integration of an electro absorption type of optical modulator and a distributed feedback laser.
The solid phase composition and the thickness of the grown layer would be varied in response to the difference of width of the mask, even if the growth conditions are the same. This means that the thickness of the selectively grown layer varies when the mask is changed in width for control of bandgap energy, even if it is not preferable.
Practically, such a quantum well structure layer is sandwiched with doped clay layers, and current is injected or electric field is applied to the well structure for providing optical function with the well layer. In this processing, there is disadvantage in that electric characteristics, such as device resistance and reverse breakdown voltage, are changing in response to the width variation of the mask and, therefore, important parameters for reliability of the optical functional device cannot be fixed. If the thicknesses of layers are not even on each portion of the substrate, that may cause the quality of photolithography processing to deteriorate.